Pressure sensor for detecting the pressure in the combustion chamber of internal-combustion engines

ABSTRACT

In a pressure sensor, the force is introduced onto a piezoresistive measuring element via a diaphragm and a punch. Since no exact mechanical adjustment between the diaphragm and the measuring element is necessary, various relatively inexpensive forms of diaphragm with different measuring sensitivity can be used. The punch and the piezoresistive measuring element can be pushed through an opening in the housing and pressed onto the diaphragm so as to provide an interlocking frictional connection between the punch and the measuring element.

BACKGROUND OF THE INVENTION

The present invention relates to a pressure sensor.

More particularly, it relates to a pressure sensor which has a housing,and a punch arranged between a diaphragm and a measuring element in thehousing to introduce the pressure onto the measuring element.

The invention also deals with a process of producing such a pressuresensor. In the case of such a pressure sensor, known from GermanOffenlegungsschrift 31 25 640.6, the piezoresistive measuring elements,such as for example thick-film resistors of cermet, contactive plasticor metal, are applied to a base. The resistor element and the base arearranged as close as possible to the pressure chamber in order to beable to determine the prevailing pressure. Furthermore, the measuringsignal is passed with the aid of electric leads to an electronicevaluation circuit arranged outside the housing of the pressure sensor.As a result, the piezoresistive elements and the electronic componentshave to be elaborately connected to one another with the aid of sheathedleads. Since the piezoresistive measuring element is exposed directly tothe pressure, it is also exposed to the high temperatures prevailing inthe combustion chamber. The flames spread there at a temperature ofabout 2000° C., whereby stresses can occur in the housing. As a result,the pressure signal is falsified by the high temperatures.

Furthermore, European Preliminary Published Specification 85 111 895.0discloses a pressure sensor in which the thick-film resistor is arrangedon the bottom of a base. However, this pressure sensor is intended onlyfor determining the pressure in distributor pumps. The high temperaturesprevailing in the combustion chamber would falsify the measuring signalin the case of this design as well.

U.S. Pat. No. 4,645,965 describes a pressure sensor of which themeasuring element consists of piezoelectric material. This piezoelectricelement is arranged in a housing which has an opening towards thecombustion chamber. This opening is closed off by a diaphragm, thepiezoelectric element being in effective connection with this diaphragm,and consequently with the pressure prevailing in the combustion chamber,via a punch. The piezoelectric element bears against a shoulder formedin the housing. The use of the piezoelectric element also gives rise toconstructional differences in comparison with the subject-matter of theapplication. In general, it is required that the measuring elementoperates potential-free. As a result, it is necessary that, in additionto the two contact discs already necessary for the piezoelectricelement, a further insulating disc is fitted between the contact discsand the counterbearing. The contacting of the piezoelectric ceramic discis possible only by elaborate welding of a wire or with the aid of arelief in the punch. If again a potential-free sensor is chosen, thiseffort would also double. Since the piezoceramic element has to be usedas a discrete component, it is also relatively expensive inconstruction. Furthermore, several bonding points, elaborate andfalsifying the measuring signal, are required. Even in a normal casewith a piezoelectric disc and two contact discs, this results in fourbonding points; with a potential-free sensor, it would be five bondingpoints. Owing to the measuring principle used as a basis, it isessential that the punch is bonded perpendicularly onto thepiezoelectric element. Piezoceramic elements have a disadvantageous,relatively high temperature response of measuring sensitivity, which maylie between 15% and 70% in the application temperature range. Aging overthe service life is typically 2% to 10%. The hysteresis of thepiezoceramic of 5% to 25% is very high and thereby significantlyrestricts the accuracy of the combustion chamber pressure measurement.In particular, however, the multilayered construction of thepiezoelectric arrangement makes relatively elaborate and complicatedfitting into the housing of the pressure sensor necessary. In the caseof piezoelectric elements, as distinct from piezoresistive elements,under the effect of pressure a charge is generated, and consequently avoltage, for producing the measuring signal. This voltage is picked upand evaluated. As distinct from this, in the case of piezoresistiveelements a voltage is applied, and the electrical resistance in thepiezoresistive element is changed by the pressure acting. Since, in thecase of this pressure sensor, the piezoelectric element and the punchare fitted from one side into the bore and the latter is closed by a potdiaphragm, the position of the pot diaphragm and of the piezoelectricelement have to be matched precisely to each other. This can be achievedonly with a pot diaphragm.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apressure sensor for detecting a pressure in a combustion chamber ofinternal combustion engine and a process of producing the same, whichavoids the disadvantages of the prior art.

In keeping with these objects and with others which will become apparenthereinafter, one feature of the present invention resides, brieflystated, in a pressure sensor having a housing and a punch arrangedbetween a diaphragm and a measuring element of the housing andintroducing a pressure onto the measuring element, wherein in accordancewith the present invention the measuring element is formed as apiezoresistive element and the diaphragm is fastened on the shank of thehousing without mechanical exact adjustment in relation to the measuringelement.

Still another feature of the present invention is a process forproducing a pressure sensor, in accordance with which a diaphragm isfastened on a housing of the pressure sensor, and a punch and apiezoresistive measuring element are pushed through a second openinginto the housing and pressed onto a diaphragm in such a way that aninterlocking frictional connection is produced between the punch and themeasuring element.

The pressure sensor designed in accordance with the present invention,has the advantage that piezoresistive measuring elements are, inprinciple, potential-free if they are realised on an insulatingsubstrate. As a result, no additional constructional measures arenecessary any longer. Furthermore, a piezoresistive measuring elementcomprises only one layer. If a thick-film resistor is used aspiezoresistive element, it can be printed on by particularly simple, andconsequently extremely inexpensive, standard techniques. In this case,printing onto the contacting conductor tracks is possible, theconnecting leads being able to be soldered onto the conductor tracks. Asdistinct from the piezoelectric effect, in the case of thepiezoresistive element no bonding points are necessary. In order toprevent slipping during assembly, there may possibly be an adhesivelayer between the substrate and the counter-bearing. Homogeneous forceintroduction onto the piezoresistive element can be achieved withoutgreat constructional effort. By simple impressing of the thick-filmresistor with a soft punch tip, a uniform force distribution over theentire loaded surface area of the thick-film resistor is possible. Noadditional elaborate adjustment is required. Piezoresistive elements,specifically cermet thick-film resistors, have a low temperatureresponse of measuring sensitivity, which is only between 5% and 10%,seen over the entire application temperature range. Aging over theservice life of the piezoresistive elements is very low and lies below2%. Furthermore, the piezoresistive elements have virtually nohysteresis; it would lie below 1%. If piezoresistive elements are used,it is possible to compensate in a simple way for the roughnessesoccurring at the end of the punch. By the use of a relatively softmaterial for the punch, but in particular by the use of a soft materialfor the end of the punch facing the piezoresistive element, a uniformforce introduction over the entire contact area is possible.

The use of a piezoresistive measuring element also allows the use ofalternative and inexpensive diaphragm designs differing from that of thepot diaphragm. There need not be any precise predetermined position.Rather, it is possible to leave matching the two parts, diaphragm andmeasuring element, to each other until fitting.

During the impressing of the glass layer surrounding the piezoresistiveelement with the punch and the setting of the prestressing, theelectrical variables of the piezoresistive measuring element and of itsevaluation electronics are measured and set to a desired nominal value.As a result, an exact trimming is possible in a Simple and inexpensiveway without mechanically exact adjustment.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section through a pressure sensor,

FIG. 2 shows a detail and

FIGS. 3 to 16 show various modifications of the form of the diaphragm,of the end of the punch and of the housing shank.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, 10 denotes the housing of a pressure sensor 11 fordetermining the pressure in the combustion chamber of aninternal-combustion engine. It has a central, continuous stepped bore12. The opening 13 of the housing 10 facing the combustion chamber isclosed off by a diaphragm 14. In FIG. 1, the diaphragm 14 is designed asa plate which is fastened on the end face 16 of the shank 15 of thehousing 10 with the aid of a welded joint 17. The diaphragm 14 is formedparticularly advantageously from a superalloy, that is to say an alloyof, for example, about 50% Ni, 20% Cr, 20% Fe. Against the middle areaof the diaphragm 14 there bears with its one end a punch 18, which withits other end bears against a piezoresistive measuring element 19.Piezoresistive measuring elements are to be understood as elements whichchange their resistance value under the effect of pressure. For this,thick-film resistors may be used for example. As materials for this,cermet, contactive plastic or metal etc. can be used. The measuringelement 19 is printed onto the base of a hybrid 22. A hybrid is normallyto be understood as a base having printed-on circuit parts, such as forexample resistors and conductor tracks etc. with semiconductors such asICs (integrated circuits), which are placed on the base and bonded tothe circuit parts, for example by bonding wires. The punch 18 itself mayconsist of glass ceramic, in order thereby to ensure a good thermalinsulation between the diaphragm, that is to say between the pressurechamber of which the pressure is to be determined, and thepiezoresistive measuring element. The end of the punch 18 facing thehybrid 22 may be conically designed, so that its end has approximatelythe diameter of the measuring element 19. This makes it possible toguide the punch 18 in the bore 12, but on the other hand to limit thearea of pressure transfer to the size of the measuring element 19. Thebase 20 of the hybrid bears furthermore against a counterbearing 23,pressed into the bore 12.

The conical design of the end of the punch 18 may be necessary in orderto match the congruent surfaces of the piezoresistive measuring element19 and of the end of the punch 18 approximately to each other. If, onthe other hand, the entire punch were to be reduced in its diameter, thepunch could possibly break off during pressure transfer. In order toensure as defined a force introduction as possible from the punch ontothe piezoresistive measuring element 19, the punch 18 is produced from arelatively soft material, at least in the end region 20. This makes itpossible for surface roughnesses of the end of the punch to be preventedfrom having the effect of measuring errors in the piezoresistivemeasuring element 19 by thus achieving a form fit between the end faceof the end of the punch and the surface of the piezoresistive measuringelement. Furthermore, it would also be conceivable to produce the entirepunch 18 from a soft material. This would, however, in turn adverselyaffect the rigidity of the punch 18. When selecting the material for thepunch 18 it must also be taken into consideration that the punch is tohave as low a thermal conductivity as possible, so that the measuringsignals are not falsified by temperature fluctuations or by anyinfluencing by the temperature itself. Glass ceramic, for example, has agood low thermal conductivity. Relatively soft metal, for example Al,brass, Cu, plastic etc. may be used for example as material for theregion 20 of the punch 18. The material for the region 20 need not bematched to the low thermal conductivity.

As well as the piezoresistive measuring element 19, on the same side ofthe base of the hybrid 22 are also the electronic components 25 of theevaluation circuit, such as for example resistors, transistors etc. Theelectronic semiconductor components 25 and the piezoresistive measuringelement 19 are connected with the aid of bonding wires 26 and thick-filmconductor tracks 27. The output of the preprocessing circuit is led viaa lead 28 to an evaluation circuit and control device (not shown) of theinternal-combustion engine. For this, the counterbearing 23 has acontinuous bore 29, which runs approximately axially parallel to thebore 12 and in which the outgoing lead 28 is guided. The lead 128 isfastened in a grommet 30 of the cover 31 closing off the bore 12. Forprotection against harmful environmental influences, such as for examplemoisture, the bore 12 is filled in the region of the piezoresistivemeasuring element 19 and of the electronic components 25 and in theregion between the counterbearing 23 and the cover 31 with a castingcompound 32.

Instead of a bore 29, a segment may also be cut out on thecounterbearing 23 for guiding through the lead 28. The counterbearing 23may also, however, be screwed into a bore 12 of the housing 10.

In FIG. 2, the design according to the invention of the piezoresistivemeasuring element is shown. A first (36) and a second conductor track 37are printed on the base of the hybrid 22 of Al₂ O₃ substrate. Printed onbetween the two conductor tracks 36, 37 is a first resistive film 38,the ends of the resistor run 38 overlapping with the ends of the twoconductor tracks 36, 37, in order to establish an electric contact.Printed onto this first resistor run 38 in a layer-like manner are aplurality of further resistor runs 39. These further resistor runs arearranged here in such a way that a kind of pyramid is produced, whichmeans that the resistors are in each case of shorter design in ascendingsequence of the layers and consequently are not congruent or do notoverlap in the edge region. The entire pyramidal arrangement of theresistor runs 38, 39 and at least the ends of the conductor tracks arecovered by a glass layer 40, produced from a glass paste, in order toachieve an electrical insulation. The end of the punch 18 bears with itsregion 20 on the glass layer 40 or on the last of the resistor runs 39in such a way that the punch 18 is placed quite centrally on thisresistor run 39 only. This means that the force is introducedexclusively in the region of the uppermost resistive film and forcebypasses are thus avoided.

For a particularly homogeneous force introduction onto thepiezoresistive element 19, the punch 18 should have a soft tip 20 of,for example, Al, brass, Cu, plastic etc. With this soft tip, the punchis to be pressed onto the piezoresistive element 19 and here inparticular onto the glass layer 40. The pressure for this should begreater than-the pressure later to be determined. For this, a multipleof the maximum pressure may be applied to the diaphragm and the punch orelse the intended position deliberately overpressed when pressing in thecounterbearing 23. This produces an impressing of the soft end of thepunch.

It is also possible, however, instead of a single piezoresistivemeasuring element of a pyramidal design, to arrange additionally afurther piezoresistive measuring element on the hybrid 22 or even 4measuring elements. In this case, these resistor elements are wired toone another in a Wheatstone half-bridge or full-bridge circuit. One ofthe two or 2 of four piezoresistive measuring elements is then subjectedto pressure by the punch 18, while the other piezoresistive measuringelement or both other measuring elements are arranged as closely aspossible in the region of the first or the two first piezoresistivemeasuring elements, but are not subjected to pressure. This makes itpossible to carry out a temperature compensation without having toprovide additional electronic components for a compensation oftemperature fluctuations in the evaluation circuit.

The various resistor runs 38, 39 may be printed in or transversely tothe direction of the current flowing through the resistor runs.

Particularly good measured values are possible by the arrangementaccording to the invention of the piezoresistive measuring element 19.Furthermore, a particularly simple assembly of the pressure sensor 11 ispossible by the arrangement of the components 25 on a hybrid 22. Thepiezoresistive element 19, the electronic components 25 and thecounterbearing 23 can be preassembled with the outgoing leads 28 as astructural unit outside the housing 10. Consequently, the components 25and the piezoresistive measuring element 19 can be checked for theirserviceability already when outside the housing 10 of the pressuresensor 11. During final assembly, consequently all that needs to be doneis to push the preassembled, already checked unit from an opening of thebore 12 into the housing with inserted punch 18, until the punch bearsagainst the diaphragm 14. Since the counterbearing 23 is pressed intothe housing 10, the unit is easily centred and fixed in the housing 10.

Since no exact mechanical adjustment is necessary between thepiezoresistive measuring element 19 and the diaphragm 14, the mostvaried modifications of the diaphragm and of the shank of the housingcan be used, as are represented in FIGS. 3 to 16. The diaphragms are inthis case welded onto the empty housing 10. Since, as distinct from theprior art, it is possible for the punch 18 and the piezoresistivemeasuring element 19 to be fitted through a second opening of thehousing, an overall matching of diaphragm, punch and piezoresistiveelement is not necessary or possible until in the final fitted state.The diaphragm can consequently be placed on the shank of the housingwith great tolerances. The plate-shaped diaphragm shown in FIG. 1 wouldrepresent the most simple solution. It has, however, no special bendingzone, so obtaining an exact measuring signal can be achieved only undercertain circumstances. According to the modification of FIG. 3, thediaphragm 45 has an annular groove 46 between the shank 16 of thehousing 10 in the wall facing the punch 18. In the region of thisannular groove 46, the diaphragm 45 is particularly easily bendable, sothat even small pressures can be detected. The diaphragm 47 of FIG. 4represents an improvement of the measuring sensitivity in comparisonwith the diaphragm 45 of FIG. 3.

Here, instead of one annular groove 46, two narrow annular grooves 48are formed next to each other. It is, of course, also possible toincrease further the number of annular grooves in order to improve themeasuring sensitivity. In the case; of the modification according toFIG. 5, the bending region is achieved by a recess 50 in the region ofthe opening of the housing 10, while, as in FIG. 1, a plate 14 is usedas diaphragm. The diaphragms 51 and 52 shown in FIGS. 6 and 7,respectively, are kinematic reversals. While in FIG. 6 the punch 18a isshortened in relation to the housing 10a, the punch 18b in FIG. 7 islengthened in relation to the housing 10b. As a result, in each case abearing region 51a or 52a is produced for the diaphragm 51 or 52,respectively, on the shank of the housing and the punch, and in betweenthese an obliquely running bending region 51b or 52b, respectively. Asin FIG. 5, the shank of the housing of FIGS. 8, 9, 10a, b, 11, 13, 15,17 again has a recess 50, in which however the diaphragm is arranged. InFIG. 8, the diaphragm 55 has in the region of the punch 18 acontinuation 56, by which it rests on the punch 18, of shortened designin relation to the shank 15. As can be seen from FIGS. 8 and 9, variouspossibilities of compensating for the different thickness of thediaphragm 55 in the region bearing on the shank 15 and on the punch 18are conceivable. In FIG. 9, the diaphragm 55 is conical in this region.In FIG. 11, the punch 18 is again of longer design than the shank 15.The diaphragm 60 has the form of a pot, but does not embrace the shankof the housing but bears against the wall 61 of the recess 50.

In FIGS. 10a, b, 12 and 13 the punch 18 terminates, approximately flushwith the bottom of the recess 50 in the shank of the housing 10. Thediaphragm 63 shown in FIGS. 10a, b has waves 64 serving as bendingregion, which waves represent spirally tangentially extendingdepressions. In FIG. 12, the bending region of the diaphragm 65 is anencircling wave 66. Depending of the height of this wave 66, thesensitivity of the diaphragm 65 can be varied. In the case of the designaccording to FIG. 13, an annular groove 67, formed in the shank 16,serves as bending region. The diaphragm 68 itself is again a plate, asalready used in the case of the exemplary embodiment according to FIG.5. The diaphragm 65 may in this case be arranged in the recess 50, orelse be welded onto the end face of the shank 15.

FIG. 14 shows a pressure sensor having a punch 69, shortened in relationto the shank 15. The diaphragm 70 is of approximately pot-shaped designand has a rising profile 71 in the bending region. The wall 72 of thediaphragm 70 bears against the inner wall of the shank 15.

The two diaphragms 75 and 78 of FIGS. 15 and 16 do not exhibit anydeformation in the actual measuring region but have resilientproperties. For this, the diaphragm 75 has, seen cross-sectionally, azig-zag region 77 in the area of the shank region protruding beyond theshank 15. In this region 77, the diaphragm 75 is of a resilient design.In FIG. 16, the diaphragm 78 has, seen cross-sectionally, an s-shapedregion 79. In this case, the punch 18 again protrudes beyond the shank15 and the diaphragm 78 is firmly welded in the recess 50.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in apressure sensor for detecting the pressure in the combustion chamber ofinternal-combustion engines, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:
 1. A process for producing a pressure sensor for detectingpressure in a combustion chamber of internal combustion engines andhaving a diaphragm, a measuring element and a punch, the processcomprising the steps of fastening a diaphragm on a housing in a regionof a first opening of the housing; and introducing a punch and apiezo-resistive measuring element through a second opening of thehousing into the housing and pressing the punch onto the diaphragm insuch a way that a form fit is produced between the punch and themeasuring element.
 2. A process for producing a pressure sensor fordetecting pressure in a combustion chamber of internal combustionengines and having a diaphragm, a measuring element and a punch, theprocess comprising the steps of fastening a diaphragm on a housing in aregion of a first opening of the housing; introducing a punch and apiezo-resistive measuring element through a second opening of thehousing into the housing and pressing onto the diaphragm in such a waythat a form fit is produced between the punch and the measuring element;and pressing a counterbearing for the punch so far into the housing thatan impression of the punch is produced on one of the piezo-resistivemeasuring elements and a glass layer surrounding the piezo-resistivemeasuring element.
 3. A process as defined in claim 2, and furthercomprising the step of placing the diaphragm onto a shank of thehousing, which shank has the first opening, so that no mechanicaladjustment is needed between the piezo-resistive measuring element andthe diaphragm.
 4. A pressure sensor for detecting pressure in acombustion chamber of internal combustion engines, comprising a housing;a diaphragm and a measuring element provided in said housing; a puncharranged between said diaphragm and said measuring element in saidhousing and introducing a pressure onto said measuring element, saidmeasuring element being formed as a piezo-resistive element, saiddiaphragm being fastened to said housing so that no mechanicaladjustment is needed between the piezo-resistive measuring element andthe diaphragm, said housing having a shank, said diaphragm beingfastened on said shank of the housing.
 5. A pressure sensor as definedin claim 4, wherein said shank of said housing has a recess, saiddiaphragm being arranged in said recess of said shank of said housing.6. A pressure sensor as defined in claim 5, wherein said shank of saidhousing has an inner wall provided with an annular groove.
 7. A pressuresensor for detecting pressure in a combustion chamber of internalcombustion engines, comprising a housing; a diaphragm and a measuringelement provided in said housing; a punch arranged between saiddiaphragm and said measuring element in said housing and introducing apressure onto said measuring element, said measuring element beingformed as a piezo-resistive element, said diaphragm being fastened tosaid housing so that no mechanical adjustment is needed between thepiezo-resistive measuring element and the diaphragm, said diaphragmbeing formed as a plate.
 8. A pressure sensor for detecting pressure ina combustion chamber of internal combustion engines, comprising ahousing; a diaphragm and a measuring element provided in said housing; apunch arranged between said diaphragm and said measuring element in saidhousing and introducing a pressure onto said measuring element, saidmeasuring element being formed as a piezo-resistive element, saiddiaphragm being fastened to said housing so that no mechanicaladjustment is needed between the piezo-resistive measuring element andthe diaphragm, said diaphragm having a side facing said punch and isprovided with at least one annular groove in said side.
 9. A pressuresensor for detecting pressure in a combustion chamber of internalcombustion engines, comprising a housing; a diaphragm and a measuringelement provided in said housing; a punch arranged between saiddiaphragm and said measuring element in said housing and introducing apressure onto said measuring element, said measuring element beingformed as a piezo-resistive element, said diaphragm being fastened tosaid housing so that no mechanical adjustment is needed between thepiezo-resistive measuring element and the diaphragm, said diaphragmhaving a bending region formed by a spiral depression.
 10. A pressuresensor for detecting pressure in a combustion chamber of internalcombustion engines, comprising a housing; a diaphragm and a measuringelement provided in said housing; a punch arranged between saiddiaphragm and said measuring element in said housing and introducing apressure onto said measuring element, said measuring element beingformed as a piezo-resistive element, said diaphragm being fastened tosaid housing so that no mechanical adjustment is needed between thepiezo-resistive measuring element and the diaphragm, said diaphragmhaving a bending region formed as a conically running section.
 11. Apressure sensor for detecting pressure in a combustion chamber ofinternal combustion engines, comprising a housing; a diaphragm and ameasuring element provided in said housing; a punch arranged betweensaid diaphragm and said measuring element in said housing andintroducing a pressure onto said measuring element, said measuringelement being formed as a piezo-resistive element, said diaphragm beingfastened to said housing so that no mechanical adjustment is neededbetween the piezo-resistive measuring element and the diaphragm, saiddiaphragm having a continuation provided in a region of said punch. 12.A pressure sensor for detecting pressure in a combustion chamber ofinternal combustion engines, comprising a housing; a diaphragm and ameasuring element provided in said housing; a punch arranged betweensaid diaphragm and said measuring element in said housing andintroducing a pressure onto said measuring element, said measuringelement being formed as a piezo-resistive element, said diaphragm beingfastened to said housing so that no mechanical adjustment is neededbetween the piezo-resistive measuring element and the diaphragm, saiddiaphragm having a bending region formed by a radially encircling wave.13. A pressure sensor for detecting pressure in a combustion chamber ofinternal combustion engines, comprising a housing; a diaphragm and ameasuring element provided in said housing; a punch arranged betweensaid diaphragm and said measuring element in said housing andintroducing a pressure onto said measuring element, said measuringelement being formed as a piezo-resistive element, said diaphragm beingfastened to said housing so that no mechanical adjustment is neededbetween the piezo-resistive measuring element and the diaphragm, saidpunch having an end, said diaphragm having a substantially S-shapedregion in an area of said end of said punch as seen in a cross-section.